quarter wavelength line

[rameshbalan]

i have one doubt, if all the lines in electronic circuits are transmission lines or, if the signal wavelength is less than the length of the cable then it becomes transmission line.
if it is the case when the signal frequency is less than the length of the line in transmission line then why quarter wavelength line also called transmission line. because here the signal frequency is greater than 4 times of transmission line. please clear my doubt.

 

[jiripolivka]

i have one doubt, if all the lines in electronic circuits are transmission lines or, if the signal wavelength is less than the length of the cable then it becomes transmission line.
if it is the case when the signal frequency is less than the length of the line in transmission line then why quarter wavelength line also called transmission line. because here the signal frequency is greater than 4 times of transmission line. please clear my doubt.

Simply taken, any transmission line can be used. If you short one end of it, then one quarter-wavelength away its impedance will be transformed into "open".
Read about the Smith diagram and learn to use it. This diagram simplifies the complex equations into an easy understandable tool showing how the line impedance vAries along the line.

The impedances shown in the Smith diagram are "longitudinal". The characteristic impedance of an line is the "transversal" impedance, and other impedances are "normalized" to it.

 

[rameshbalan]

i have one doubt, is all the lines in electronic circuits are transmission lines or, is the signal wavelength is less than the length of the cable thenonly it becomes transmission line?. what is the condition that the line to become transmission line.
if it is the condition for transmission line that when the signal frequency is less than the length of the line then why quarter wavelength line also called transmission line. because here the signal frequency is greater than 4 times of transmission line. please clear my doubt.

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sorry i made mistake in early question, this is my question.
i have one doubt, is all the lines in electronic circuits are transmission lines or, is the signal wavelength is less than the length of the cable then only it becomes transmission line?. what is the condition that the line to become transmission line.
if it is the condition for transmission line that when the signal frequency is less than the length of the line then why quarter wavelength line also called transmission line. because here the signal frequency is greater than 4 times of transmission line. please clear my doubt.

 

[WimRFP]

A transmission line is a transmission line (coaxial cable, trace on PCB), the signal frequency does not change that. I think your question relates to whether or not you need transmission line theory to explain the behavior of a piece of transmission line.

As a rough guideline you don't need transmission line theoriy when the electrical length of the line is less then 0.1 lambda for the highest frequency. The electrical length is the length based on the actual propagation delay. As waves in dielectric material go slower, the electrical length is more then the physical length. For example coaxial cable with solid (non-foam) PE dielectric has a velocity factor of around 0.66. So the propagation speed is 0.66*Co (Co = speed of light in free space).

When physical length < 0.1 lambda, you can use standard formulas for capacitance and inductance together with regular AC network theory to calculate the effect of a transmision line on signal propagation or transfer. The transmission line is then assumed to be a "lumped component" as the laws of Kirchhoff are used to solve the problem.

when physical length > 0.1 lambda, the error by using regular Kirchhoff based AC network theory becomes very large with increasing length, and then you need transmission line theory to solve a problem.

As systems become faster (shorter rise and fall time, higher frequencies, etc), somebody working in the electroncs field need to understand transmission lines. So it is good to understand the Smith Chart and time domain behavior of transmission lines.

 

[biff44]

I think someone filled your head with bad information. Any circuit with a transmission line in it can be properly analyzed with transmission line theory. For example, if you had a transmission line that was 1/100th of a wavelength in physical length...you could still analyze it as a transmission line.

What they should have clearly told you was that if the length of line is much smaller than a wavelength, such as << 1/10th of a wavelength, then you can analyze the circuit with the approximation that it is a lumped element circuit. For instance, you could replace a very short high impedance line with a series inductor of the right value, and approximate the correct response. Similarly a very short low impedance transmission line can be approximated by a shunt capacitance.